The best way I ever found to trap them is by taking a glass jar and laying it on it's side on the substrate. Then add an algae wafer. It won't take long usually for the snails to come running. When you get some in there just gently right the jar, remove it from the tank, remove the snails, and put the jar and wafer back in. Just be sure not to put it in at night and get it out in the morning as usually the snails have feasted all night and crawled back into their hidey holes to digest.